Why doesn't this boot loader code work?

My expectation is that it prints a string, but nothing is printed out. When I make the string shorter, it sometimes works, and when I make them longer again, it works sometimes.

I don't know why this isn't working.

Could somebody help me? Thanks.

The assembly code I'm using is:

(Emacs 23, Ubuntu 10.10, nasm, VirtualBox OSE)

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
org 0x7c00
bits 16
str:
    db "Some say the world will end in fire",10,13
    db "Some say i开发者_Python百科n ice",10,13
    db "From what I've tasted of desire",10,13
    db "I hold with those who favor fire",10,13
    db "But if I had to perish twice,",10,13
    db "I think I know enough of hate",10,13
    db "To say that for destruction ice",10,13
    db "is also great and would suffice."
    db "Robert Frost - Fire and Ice"
    db 0
start:
    xor ax,ax
    mov ds,ax
    mov es,ax
    mov si, str
    xor bx,bx
    mov ah, 0x0e
print:
    lodsb   ;al = current char
    cmp al, 0
    je end
    int 0x10
    jmp print
end:    
    cli
    hlt

    times 510 - ($-$$) db 0
    dw 0xAA55
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

Because it starts executing code right at the instruction at 7c00. That, unfortunately is where you have your string.

You should precede that string with a jmp instruction so that it jumps to start.

This is usually a short jump EB xx followed by a NOP 90. Some BIOS' may insist on that being of this form even though it doesn't really matter to the processor.

In other words, you'd be looking for something like:

org 0x7c00
bits 16
realstart:
    jmp short start
    nop
str:
    db "Some say the world will end in fire",10,13
    :
    db "Robert Frost - Fire and Ice"
    db 0
start:
    xor  ax,ax
    :

Just keep in mind that the short jump is limited as to what distance it can go, roughly +/-128 bytes, so your string size will be necessarily limited by that. If your BIOS doesn't require the EB xx 90 format, you can just do a regular jump.

The other thing you could try is to move the entire string to after the hlt instruction:

org 0x7c00
bits 16
start:
    xor  ax,ax
    :
end:    
    cli
    hlt
str:
    db "Some say the world will end in fire",10,13
    :
    db "Robert Frost - Fire and Ice"
    db 0

but, again, this depends on your BIOS not requiring the jmp/nop combo at the start.

A good way to verify that paxdiablo and Igor Skochinsky are correct is to put the text string in a file, then run it thorough a disassembler. The shorter strings that print properly should disassemble into a code string that doesn't hurt anything. The shorter strings that fail, and the longer string will either include illegal instructions, jump or call instructions, or even just have a 2- or 3-byte instruction at the end that eats up the opcode for the "xor ax,ax" instruction at the beginning of your code.